repository_ctx.sha256 for safer repository rules

[pcj]

Repository context should expose a sha256 function that returns a sha256 string for a given path file artifact. Makes it easier to write safer repository rules. Please indicate if a PR for such a function would be welcome.

[damienmg]

That makes sense to me but we need to do an API review first. However the full skylark repository hasn't gone through such an API review yet. I am cleaning it as a first step and then we can discuss this further.

[kchodorow]

This is probably more easily solvable via #1410. See also #991 (closed in favor of #1410).

[pcj]

Could also be addressed via allowing FileURLConnection's (file urls) in ctx.download, at which it becomes a generic cp with sha256 checking capability.

[jin]

I believe this is now handled by @aehlig's work on repository rule return values and bazel sync.